Deuteronomy 22:9
Thou shalt not sow thy vineyard with divers seeds: lest the fruit of thy seed which thou hast sown, and the fruit of thy vineyard, be defiled. Thou shalt not sow thy vineyard with divers seeds: lest the fruit of thy seed which thou hast sown, and the fruit of thy vineyard, be defiled.
In Deuteronomy 18-26 stands special instruction and direction given through Moses, repeating laws for the government of Israel and the regulation of worship and containing great principles for deciding questions. The command forbids sowing a vineyard with different kinds of seed together, because the mixed planting defiles or damages both the fruit of the seed sown and the fruit of the vineyard. The harm is seen in agriculture, where wheat and oats sown together ruin the oats and injure the wheat.
The rule about the seed sown in the field was part of teaching embedded in ordinary work, so that cultivating the soil carried lessons pointing to a holy life and to the Redeemer who would take away the sins of the world. It belongs to a broader lesson against incongruous mixtures, teaching that unlike things should not be mingled in ways that dull moral sensibility.
God's people must therefore keep distinct from the world, whose nature and motives differ from theirs, since such a union benefits neither side. Faithfulness to God requires avoiding evil companions and corrupting influences, whose association corrupts. Just as wheat is injured when mixed with other seed, even spiritually minded people can be led astray through such association.